Translatable display sign

ABSTRACT

A carriage or series of carriages carrying electromagnetically actuable display elements is moved intermittently in a path. At a location in the path magnetizing means exterior to the carriage is used to magnetize carriage-mounted control elements for the display elements, which control elements act to hold the display elements in a desired position until the control element are differently magnetized.

United States Patent [56] References Cited UNITED STATES PATENTS 8/1966 Levy et al.

[72] Inventor CharlesN. Smith Cooksville, Ontario, Canada Appl. No. 882,106

[22] Filed Dec. 4, 1969 [45] Patented Nov. 30, 1971 [73] Assignee Ferranti-Packard Limited Toronto, Ontario, Canada Assistant Examiner-Michael Slobasky Attorney-Weston & Hanley [54] TRANSLATABLE DISPLAY SIGN ABSTRACT: A carriage or series of carriages carrying elecl8 Claims, 10 Drawing Figs.

tromagnetically actuable display elements is moved intermit- 52 us. 340/373, lenlly in a p A! a location n the p h m gn izing means exterior to the carriage is used to magnetize carriage-mounted control elements for the display elements, which control elements act to hold the display elements in a desired position until the control element are differently magnetized.

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Inventor CHARLES N. SMITH Wweumw TRANSLATABLE DISPLAY SIGN This invention relates to a translating display apparatus wherein magnet-carrying elements movable between two positions are designed to provide two mutually contrasting visual indications in a forward or viewing direction transverse to the translation direction corresponding to the two positions. The position of each element is determined by a pole piece translating with the element but exterior thereto and permanently reversibly magnetizable, and of a strength and location so that its sense of magnetization controls the orientation of the element carried magnet. Means are provided in a predetermined location along the translation path for magnetizing the pole piece ot" an element or the pole pieces of an array of elements, which are then located at such location.

Due to the magnetization of the pole pieces, the desired indication or display is created at the predetennined location but, due to the remanent flux of the permanently magnetizable member, the elements are held in the desired indication or display position, after they are moved away from the location where the pole pieces are energized.

I use "transverse herein to indicate a direction across or athwart a datum direction, that is, not necessarily geometrically perpendicular to such datum direction, but having a large component along such perpendicular.

The elements shown are similar in principle and in operation to those described in my copending U.S. application Ser. No. 793,401 filed Jan. 23, 1969, now U.S. Pat. No. 3,537,197. This type of element is thought to be most suitable for use with this invention.

However it will be noted that the elements instead of being of the lever type, as shown, could be of the disc type as shown in U.S. Pat. No. 3,140,553 issued July 14, i964; or U.S. Pat. No. 3,295,238 issued Jan. 3, 1967; or could be of the log type as shown in U.S. Pat. No. 3,444,551 issued May l3, 1969 or could be of the other types shown in copending application Ser. No. 592,178 filed Apr. ll, 1969, now U.S. Pat. No. 3,469,258. All the heretofore-referred-to patents or applications are assigned to Ferranti-Packard Limited. in each case, the movable element carries a permanent magnet, movable therewith, oriented and located so that the influence thereon of an exterior magnetic field will move the element between its two positions.

Thus the invention includes means for translating a carriage carrying such an array in a path transverse relative to the viewing direction to a position where a write head may supply the necessary selective magnetizing forces to cause selected elements in the positioned carriage to assume the necessary orientation to provide the necessary display. The element arrangement and design used on the carriage may be similar intrinsically to any of the arrays shown in the prior patents with the exception that, for magnetization from outside the carriage, the magnet-carrying element will be controlled by single rather than dual pole pieces, so that the pole piece will have one end projecting in a direction and at a location to control the position of its corresponding element and the other end adjacent and in some embodiments projecting from the side of the carriage opposite to that on which the elements are to be viewed, so that the latter pole piece is located so that it may be magnetized by what may be termed either as magnetic coupling" or an extension of its magnetic circuit to a properly located field exterior to the carriage. It is noted that the type of display element and its mode of operation are not confined by the invention but, on the other hand, that single pole piece actuation is sufficiently preferably that those parts of the previously referred to applications or patents, which deal with double pole piece arrangements, may be ignored in considering alternatives for use in this application.

A number of methods are discussed of performing the magnetization of the pole pieces by a field exterior to the element.

in one of these, each pole piece, corresponding to an array element, extends rearwardly of the carriage so that the number of pole pieces which may be magnetized at once (noting that there is individual selection in the case of each pole piece) extend rearwardly as an array and it will be understood that this rearward projection is substantially transverse to the translation direction. A write" head comprising a helically wound coil corresponding to each pole piece with the coils arranged as the mirror image of an array and with each coil helix defining a bore to receive the rearwardly projecting pole piece, is movable reciprocally from a position clear of the rearwardly projecting pole pieces to a position where the coil would encompass the pole pieces, so that the projecting portions of the pole pieces, in effect, form the cores for the coil. With the pole pieces forming cores for the coils, each of the latter may then be pulsed in one of two directions to provide corresponding alternative magnetization to the corresponding pole piece to produce corresponding actuation of the actuable elements. it will be noted that, if each coil is used to magnetize in either selected sense, no erase means is required when it is desired to replace one indication by another. Alternatively, instead of supplying pulses in opposite senses through the coil, there may be provided erase means, located to operate on the pole pieces of a carriage, after the purposes of a display have been served, to magnetize all of said pole pieces in a known, and usually but not necessarily the same sense; and with this arrangement and with a mirror image array of coils to perform the write" operation magnetization, pulsing for each pole piece is only required in a sense opposite to that provided by the erase and then only in the pole pieces selected for switching.

It will be obvious that, instead of moving all the coils forwardly and only energizing some, that it is possible only to move forwardly those coils which correspond to pole pieces to be energized. In many cases, however, this will be found more expensive and inconvenient since it requires individual mechanical mounting and actuation for each of the many coils in an array. However it is noted that with individual mechanical mounting and with an independent erase location, permanent magnets individually actuated and individually corresponding to the pole pieces, may be used in substitution for the coils above described. in such arrangement, the elements would all be erased in one sense (although not necessarily all in the same sense) and permanent magnets moved forwardly and rearwardly corresponding to those elements which it was desired to switch.

Alternative embodiments are available with the translating carriage. In one of these, the extension of the polepieces rearwardly from the carriage may be shortened or made substantially nil so that the pole pieces end substantially flush with the rear of the carriage. The write head is provided with a coil corresponding to each pole piece; but, in distinction to the prior embodiment, eachcoil is provided with a core, extending sufficiently along the coil toward the position of the pole piece and that, on reciprocating movement of the write head toward the carriage, the cores may be brought into contact (or near contact) with their corresponding pole piece ends for magnetic coupling therewith and magnetization of the pole piece. Contact between the core end and the pole piece need not actually take place, as long as the proximity is such that sufficient magnetic coupling is present to produce the desired magnetization. Most commonly, all cores (with the coils surrounding them) in a write head, will together be brought into magnetic coupling with the corresponding pole pieces (although only selected ones of the core endings are pulsed). Alternatively, at the cost of more complex mechanical design, the coils and cores of the write head may be selectively moved toward and away from the carriage and all those moved toward the carriage will be pulsed. While it is not possible to define the proximity required having regard to variations in core material and dimensions, it will be appreciated that the strength of flux must be such that the reduction in flux through flux leakage or reluctance of the airgap is not sufiicient to prevent the desired magnetization of the pole piece.

in another embodiment a cored coil is again used on a write head but instead of being moved reciprocally either individually or with other coils on a write head, toward and away from a carriage, the array of cored coils is maintained in position and the carriage pole pieces are merely translated into sufficientrespective proximity to their respective cored coils for the necessary magnetic coupling. It will be noted that, in this embodiment, magnetization may be obtained without the necessity of reciprocation of the write coils, and thus the overall design of the equipment, is facilitated because a small clearance between the coil cores and the carriage pole pieces can be tolerated.

It will be obvious that permanent magnets cannot be used with the nonreciprocating write head.

In drawings which illustrate a preferred embodiment of the invention:

FIGS. la and lb show the relative movements of a reciprocating, uncored energizing coil in the write" operation for the corresponding pole piece;

FIGS. 2a and 2b show the movement of an energizing coil and core relative to a carriage pole piece before and during the write" operation;

FIG. 3 shows a write operation performed with a stationary write" head;

FIG. 4 shows a perspective view of a device employing the alternative of FIGS. la and lb;

FIG. 5 is a view of a write" head;

FIG. 6 is an enlargement of a portion of FIG. 5 showing a detail of the write head energizing coil;

FIG. 7 is a rear view of a carriage showing the rearward projections of the pole pieces therefrom; and

FIG. 8 is a cutaway view of the carriage showing the opera tion of the individual members.

FIGS. la and 1b schematically indicate a write operation for a single coil and element, using an energizing coil 10 which is located to move reciprocally on and off a pole piece 12 projecting rearwardly from a carriage, at a predetermined location of the latter in a path transverse to the reciprocation direction of the coil. FIG. la shows the positions when the reciprocable coil 10 is in its retracted position clear of the pole piece 12 mounted on a carriage l5 movable, intermittently in the direction of the vertical arrow. The pole piece 12 may be considered as the one which has just moved into position in the translating direction as shown by the vertical arrow; or, alternatively, may be considered as a pole piece which has just been magnetized and which, now clear of the coil, is about to move upwardly away from the write coil 10, in the translation direction, to make room for another pole piece. FIG. 1b shows the forward position of the coil surrounding pole piece 12 which is then stationary, and at this point magnetization of pole piece 12 may be achieved if such magnetization is required in accord with the pattern to be displayed. Pole piece 12 is mounted on a carriage 15 which, as hereinafter described is intermittently translatable in the direction of the vertical arrow.

The electromagnetic theory of and the effects of the write operation are not described herein as they form no part of the presentinvention and are available to those skilled in the art through the disclosures in the Ferranti-Packard Limited patents and applications previously referred to.

Similarly, with the cored coil in FIG. 2a, the coil 14 surrounds and moves with a core 16 projecting slightly therefrom, and magnetization is performed by a magnetic coupling between the abutting ends of core 16 and pole piece 18. It will be noted that, in this position, the carriage will be moving pole piece 18 in the direction of the vertical arrow either into or out of the position shown. In FIG. 2b the pole piece will be stationary, while the coil core 16 is moved forward into abutting contact with the pole piece 18 for magnetization of the latter if the coil is pulsed. It will be noted that the rearward projection of the pole piece from the carriage may be made much shorter than with the embodiments of FIGS. la and 1b and indeed, if desired, the pole piece could end flush with the rearward wall of the carriage. Although contact is here shown, it will be noted that if a slight gap is present this will not effect the operation it being noted that sufficient flux must be transferred through the magnetic circuit including the core 16 of the coil and the pole piece 18 and that the gap acts as a reluctance reducing the flux and allowing leakage. Thus an increase in the gap increases the amperetum requirement for the coil. Referring again to FIGS. 20 and 2b it will be noted that if the pole piece is always brought to the write position in a given magnetization so that only one sense of write" magnetization is required, a permanent magnet may be used instead of the cored coil if individual mechanical actuation of the pennanent magnets is used. In such an arrangement, the magnet is oriented to magnetize the pole piece in the opposite sense to the given magnetization and are selectively mechanically activated into magnetizing proximity to the pole piece if the opposite magnetization is desired, but left remote from the pole piece if the given magnetization is desired. As previously stated this requires individual mechanical operations for each magnetic element and hence is not considered as economically feasible in most applications. Pole piece 18 is mounted on a carriage I5, movable as described in connection with FIGS. Ia and lb.

In FIG. 3 is shown the use of a coil 24 and core 26 designed and located to beseparated by a small gap from the pole piece 28 on a carriage 29 located in the write position, so that the core 26 and coil 24 not be reciprocated but may remain stationary. The pole pieces 18 are moved in a translation direction therepast being stopped in registration with the corresponding cores 26 for pulsing (or not) and thereafter pole pieces 28 and carriage 29 are moved out of registration to be succeeded by a further pole piece positioned for the write operation. In all of the applications from la to 3 it will be understood that while a single pole piece and a single magnetizing means therefore is shown, that in fact it will be customary to write" simultaneously an array of these pole pieces with a corresponding array of magnetizing means with all such pole pieces mounted on the same carriage. The array will customarily comprise 35 elements arranged in five columns and seven rows. In such arrangement, each of the elements will be provided with a pole piece having a rearwardly directed free end and, in the write position, a write element either of one of the types shown in FIGS. la-3 or a permanent magnet, as described will be provided corresponding to each pole piece in the array.

Registration of corresponding cores and pole pieces in the embodiments of FIGS. 2 and 3 will only substantially exact, but in many cases it has been found desirable to make the core of larger diameter to ensure its core end will be opposite all parts of the pole piece end.

Operation of an element in an array is demonstrated in FIG. 8 but is more fully shown in application Ser. No. 793,401 filed Jan. 23, 1969, wherein a display module provides shutters shown as 36 in FIG. 8 of this application nearly perpendicular to the viewing direction and lever-mounted elements 34 being each individually magnetically actuable to move back and forth through slots separating the shutters, between a position where they are exposed in the viewing direction and a position where they are occluded. The elements 34 are contrastingly colored to the exposed surfaces of the shutters whereby the display is created by the correct selection of the elements displayed and occluded. Each of the elements comprises a lever 30 pivotally mounted at 32 on the carriage and having at its forward end a contrasting thin element 34, here (an octagonal dot is preferred) which could as easily have been round or square or other shape, oriented in relation tothe spacing of shutters 36 to allow movement of said thin elements 34 inwardly and outwardly through said slots so that when selectively operated and contrasted with shutter surfaces of contrasting colors, letters may be produced as illustrated in the bottom row of FIG. 4.

Mounted on the lever 30 on the other side of pivot 32 from laminar element 34 is magnet 38. The magnet is oriented so that halfway between the two limiting positions of lever 30 the north-south axis of the magnet 38 extends approximately parallel to the viewing direction. Limits for movement of the lever 30 are set, for movement toward the exposed position of element 34 by the contact between the arm of lever 30 and the upper end of the shutter 36 impeding its arc of movement; and, for movement toward the occluded position of element 34, by any desired stop means such as the stop 40. The lever is designed, relative to the two terminating positions, so that at one position the outward surface of element 34 is totally exposed and at the other position the outward surface is totally occluded. The display is created and returned due to the fact that the outer surface of each element 34 is of contrasting visual appearance to the outer surface of each shutter 36. A pole piece 12, corresponding to each element, is located on and passing through the rearward wall of the casing, by gluing or the like, and having its forwardly projecting end as near as possible to the magnet 38 in its closest approach thereto and defining an axis parallel to that of magnet 38 in its median position. It will be seen that when the pole piece 12 is magnetized in one sense, that the magnet 38 is attracted and element 34 is moved to an occluded position and when the pole piece 12 is magnetized in the other sense the magnet 38 is repelled and the element 34 moved to the exposed position.

The embodiment to be described in detail uses the magnetizing method of FIGS. la-lb and pole pieces 12 therefore project rearwardly therefrom to be surrounded by the forward movement of coils 10. From FIG 8 and FIG. 7 it will be seen that the pole pieces 12 project rearwardly through the rear wall 44 of the carriage I5 and it will be noted that the write" operation shown will be similar to that shown in FIGS. la and lb. FIG. 5 is an overall view and FIG. 6 is a detailed view showing a write head 68 having bores to receive the rearwardly projecting pole pieces of FIG. 7 so that the write head may be moved on to the rearward projections of the pole pieces for the write" operation. As shown in FIG. 6 the bores in the helical winding of a helical energizing coil I0 are each of sufficient internal diameter to surround a pole piece 12 after movement theretowards. The coils are supported in any conventional manner in bores in carriage 15 If no separate erasing means is provided, coils will be selectively energized for magnetization of the device in opposite senses. If separate erasing means are provided then only those pole pieces to be magnetized in the opposite sense to that provided by the erase need be energized, and they only in the opposite sense to that provided by the erase means.

FIG. 4 shows a particular embodiment of the invention using the device of FIGS. 5-8 and the methods of FIG. 1a and lb. As shown in FIG. 4 a plurality of the individual carriages indicated in FIG. 7 are pivotally joined together by pins 48 to form a continuous chain and a plurality of these chains are provided in side-by-side relationship. It will be seen from FIG. 4 that there is no limit to the width of the display or to the number of carriages displayed. Pins 48 project on each side of the carriages 15 they join and an endless chain of carriages, as shown, is designed to be suspended between the sprocket wheels 50 and 52 which are pivotally mounted on the casing 54 by means not shown. Sprocket wheels 50 and 52 of equal diameter are, on each disc, provided with six equiangularly spaced concave outward sockets 60 to support pins therein extended between the disc pairs, and with the sprockets spaced so that the adjacent pins 48 on the belt will contemporaneously seat in adjacent sockets about the discs periphery.

It will be noted that due to the existence of the sockets 50, 52, the sprocket arrangement varies the width between the forward and rearward flights of the belt of carriage as the chain rotates. Thus in the position shown, the belt is at its narrowest, while on an advance of 30 of the sprocket wheel, the belt will be spread the diameter of the sprocket wheels measured at the sprocket grooves 60. Although not an advantage, this forward and rearward oscillation of the elements does not interfere with the effectiveness of the display and is not substantially noticeable. Moreover since the sprocket wheels are always in the same predetermined position at the time of application of a write head to the pole pieces of a carriage, the inward and outward location of the belt at this moment is always determined and does not interfere with proper operation and thus this motion does not interfere with the writing" of a display. The mechanism as described, is mounted in a housing 54 by means, not shown, leaving a viewing aperture through which the desired length and width of the display may be viewed looking from the forward right in FIG. 4. The sprocket grooves 60 in each disc of the lower sprocket wheel 52 have small slots 64 extending radially inwardly therefrom along radii for a purpose to be now described.

Any number of ways are available to those skilled in the art of achieving registration of the translatable pole pieces rearwardly projecting from a carriage with a reciprocal write head and the means shown is merely a preferred embodiment. In the embodiment shown, the write head 68, as shown in FIG. 5 is supported from the housing by means not shown on a parallelogram link 66 with the parallelogram linkage being partially pivotally connected to the frame at its upper end and suspending the write head 68 at its lower end so that a parallelogram with pivotally connected sides formed by links 66 as two opposite sides and the frame and write head between the link pivots as the other two opposite sides so that in accord with the movement dictated by the parallelogram, the write head will reciprocate onto and off of the pole pieces and since the motion of the write head is not linear in the reciprocating direction, but includes an upward and downward component,

it will be noted that this is minimized by making the parallel arms as long as possible and by having the initial entry of coil 10 about pole pieces 12 (relatively) in a direction parallel to the projection direction of the pole pieces 12, hence this entry should be made while the parallel linkage (in the embodiment shown) is at its lowest extension since the rearward extension of the pole pieces in the embodiment shown is horizontal. Pivotally attached to one of the parallel links 66 is one end of a link 70 carrying at its other end a pin 72 adapted to enter a radial slot 64 in the sprocket wheel with the pin also projecting through a vertical slot 73 in the housing for manual operation or for automatic operation by means not shown. The write" position for the carriages 44 on a belt (which will be at the lowest visible line in the array) is arranged so that at the location where the write head 68 will pass its coils about the pole pieces of the carriage to be written," the radial slot 64 will be aligned with slot 73. The wheel 52 may thus for the write operation, be approximately aligned with this position. With the wheel approximately aligned in this position the pin 72 may be lowered by means which are shown as manual but will in practice, usually be automatic and will strike the concavities defined by the adjacent upwardly located sprocket groove 60. The positioning provided by the drive will be depressed to be sufficiently accurate that the pin 72 will enter the appropriate concavities. The concavities of groove 60 define tapering sides and hence, with continued movement of the pin more deeply into the groove, the discs of wheel 52 are rotated until the pin 72 enters slot 64 achieving a predetermined positioning of the wheel 52. The belt and sprocket 50 are therefore correctly positioned by the pin 72 hence the carriage 68, in the lowest visible position, is properly positioned for reception of the write head bores over its pole pieces 12. If, as is expected, power drive means are used for the chain, the initial movement of the pin 72 downward may be used to declutch the drive from whichever of the sprocket wheels 50 or 52 is used as the drive wheel. As the downward movement of the pin 72 continues to the bottom of slot 64, the write head 68 is moved so that its coil bores surround the pole pieces of the carriage 44. The coils 24 may then be selectively pulsed, as planned. The magnetization of the pole pieces is then completed and by the movement of pin 72 upwardly in slot 73 and the movement of the parallelogram, controlled by lever 70 of the write" head 68 is withdrawn from the pole pieces 12 of the positioned carriage. At the same time the pin 72 moves out of the slot 64 releasing wheel 52 (hence both wheels and the carriage chain), for movement of the wheels 50 and 52 and the carriage chain to a new position. If reclutching for further drive of one of the sprocket wheels is required, this may be derived in accord with any one of an infinity of control techniques, from the upward motion of pin 72. Thereafter the carriage may be moved so that the forward flight of the belt moves upwardly to move the next carriage into position for cooperation with the write" head.

It will be noted that a plurality of belts as shown in FIG. 4 for displaying an extensive test, are arranged so that the carriages 44 in the same belt form a column, and carriages in the same positions on different belts form a row. Since a row will contain information to be read as a single line of printing it will be suitable if the row is written at a single step. Thus, all write heads 68 for a horizontally disposed set of belts may be ganged. For this reason and for simplicity of construction and drive, it will also be noted that all chains arranged side by side may be ganged and driven together to the design extent that all chains could if desired be unitarily designed so that all become one chain.

The model disclosed is contemplated asbeing used as a stock indicator although it could equivalently be used for providing news or any other form of information. In its application for reporting information or stocks it is considered es sential that the information be first written on a visible line in the viewing direction hence for maximum use of the area, the writing takes place in the bottom row of the array. Once a row is written, the information moves up unchanged step by step as following lines are written at the lowest visible row. Thus it will be seen that as much text may be displayed as desired. The remanent flux of the pole pieces maintains the letters in position until erased. In the preferred embodiment, the erasing is performed by a magnet 71, biased into sliding contact with the pole pieces and by any desired means and here shown as a leaf spring 75 mounted on the frame resiliently biasing the magnet 71 into position. The magnet is therefore located to brush the rearward projections of the pole pieces and strength to magnetize them all in a predetermined sense. Since the magnet 71 is permanent it will ensure magnetization of all the pole pieces over their previous magnetization in one sense presumably producing all black or all white indications from the leverm'ounted elements 34. Thus, it will be seen, that with this method of erasure, energization of the pole pieces and the pole piece windings in both senses is not required and that only those elements to be changed from their erasure position must be energized (and all in one sense only) to produce magnetization in the corresponding pole pieces in a sense opposite to that produced by the erasing magnet 38. it will be appreciated that with the method of FIGS. 2a and 2b that the same motion of the write head may be used, noting that the travel thereof may be much less since the coil 10 does not have to move a shorter distance from a location clear of its corresponding pole piece to a location where magnetization by energization of the coil may take place.

It will be noted that the arrangement of FIG. 3 does not require the special mounting for the write head since the write head is stationary. However registration means may be required operable as the pin 72 moving in slot 64 does in FIG. fl to ensure that proper alignment exists between abutting ends of cores and pole pieces before magnetization takes place.

I claim:

1. An electromagnetic display device including:

a carriage designed for movement in a predetermined translatory direction; at least one electromagnetically actuable element thereon; each such element being mounted and designed to be magnetically actuable to move between two positions, and being designed to provide, in a forward direction transverse to said translatory direction, visually contrasting appearances in said two positions; a permanently reversably magnetizable pole piece, cor-- responding to each said element and exterior thereto,

each said pole piece being mounted on said carriage, extending between an end located to control said magnetically actuable element, and an end proximate the rearward side of said carriage;

means for moving said carriage in said translatory direction to and from a predetermined position;

means, exterior to said carriage. operable to magnetize said pole piece in at least one sense;

when said carriage is in said predetermined location;

2. A device as claimed in claim 1 wherein said carriage mounts a plurality of said elements arranged in an array.

3. A device as claimed in claim 1 wherein said means exterior to said carriage is a write head, designed and connected to provide magnetic flux in a selected sense to selected ones of said elements.

4. A device as claimed in claim 1 wherein the rearward end of each said pole piece projects rearwardly from the carriage in a predetermined direction and wherein a write" head is reciprocally movable between an extended and a retracted position relative to said carriage and carries an energizing coil corresponding to each said pole piece arranged, mounted and designed so that on location of said write" head at its extended position and with said carriage in said predetermined position, an energizing coil surrounds at least some of said permanently magnetizable pole pieces and a location of said write" head at its retracted position, clears the end of said permanently magnetizable pole piece to allow movement of said pole piece in a translation direction.

5. A device as claimed in claim 1 wherein said write head is reciprocally mounted to move between an advanced and a retracted position relative to said carriage and mounts an energizing coil corresponding to each said pole piece, each said coil surrounding a core projecting therefrom in the direction of the advanced position, and wherein said write head and core are arranged so that on movement of the carriage to the forward position, flux generated in a write" head core will magnetize the corresponding carriage pole piece.

6. A device as claimed in claim 5 wherein said write head core is formed of reversibly permanently magnetizable material.

7. A device as claimed in claim 5 wherein the write head mounts cores corresponding to each of said pole pieces and arranged so that in said predetermined position, each of said write head cores are located in close proximity to the respective pole pieces, whereby selective energization of certain of said coils in a predetermined sense creates flux in the corresponding cores and wherein the magnetic coupling between a corresponding core and pole piece is such that a substantial portion of flux from a write head core proceeds into the corresponding pole piece.

8. A movable display including a carriage movable in a translation direction and mounting an array of elements each selectively movable by a corresponding magnetic field exterior to the element between two positions, said elements being adapted to provide, in a viewing direction transverse to said translation direction, contrasting visual effects, depending on which of the two positions is assumed by the element, a pole piece corresponding to each said element and exterior thereto, said pole piece being mounted on the carriage and having an end directed to provide a field at the corresponding element, and having a free end adjacent the side of said carriage opposite from that wherein viewing takes place;

means for stopping said carriage at a predetermined position, and means, exterior to said carriage, when said carriage is stopped, operable at said predetermined position, for magnetizing selected ones of said pole pieces in a predetermined sense. I

9. Means as claimed in claim 8 wherein means are provided to cause said carriages to move in a closed path and wherein, at a location remote from the said write head, means are providedfor ensuring that all pole pieces are magnetized in a predetermined sense.

10. A movable display including:

a carriage movable in the translation direction;

and mounting an array of elements, each movable by a magnetic field exterior to the element between two positions; presentive in a forward direction, transverse to the viewing direction,

said array, by selective positioning of the elements, being adapted to present said forward direction, contrasting visual effects, depending on which of the two positions is assumed by individual elements in the array;

a pole piece mounted on said carriage corresponding to each said element, and having a free end adjacent the side of said carriage opposite to that where said elements are viewed;

means for stopping said carriage in a predetermined position; and

means, exterior to said carriage, operable at said predetermined position, for magnetizing selected ones of said pole pieces in a predetermined sense.

11. Means as claimed in claim 4, wherein means are provided to cause said carriages to move in a closed path.

12. Means as claimed in claim 4'wherein means are pro- ''vided at a location spaced in the translation direction from said exterior magnetizing means, for ensuring that all said pole pieces are magnetized in a predetermined sense.

13. Means as claimed in claim 5 wherein means are provided to cause said carriages to move in a closed path.

14. Means as claimed in claim 8 wherein means are provided to cause said carriages to move in a closed path.

15. Means as claimed in claim 5 wherein means are provided at a location spaced in the translation direction from said exterior magnetizing means, for ensuring that all said pole pieces are magnetized in a predetermined sense.

16. Means as claimed in claim 8 wherein means are provided, at a location spaced in the translation direction from said exterior magnetizing means, for ensuring that all said pole pieces are magnetized in a predetermined sense.

17. Means as claimed in claim 9 wherein means are provided, at a location spaced in the translation direction from said exterior magnetizing means, for ensuring that all said pole pieces are magnetized in a predetermined sense.

18. Translating display apparatus using a carriage carrying a plurality of indication or display magnet-carrying elements, each movable with their magnets between two positions and designed to provide alternative contrasting visual indications corresponding to the two positions in a forward or viewing direction transverse to the translation direction, wherein the position from time to time of each element is determined by a corresponding pole piece mounted to translate with the element but exterior thereto, said pole pieces being permanently reversibly magnetizable and located to determine the position of the element in accord with the pole piece magnetization, and means for translating said element and pole piece in the translation direction, in combination with means, operable, at a predetermined location in the translation path, to magnetize, in a predetermined sense, the selected ones of said pole pieces.

* i I i 1 

1. An electromagnetic display device including: a carriage designed for movement in a predetermined translatory direction; at least one electromagnetically actuable element thereon; each such element being mounted and designed to be magnetically actuable to move between two positions, and being designed to provide, in a forward direction transverse to said translatory direction, visually contrasting appearances in said two positions; a permanently reversably magnetizable pole piece, corresponding to each said element and exterior thereto, each said pole piece being mounted on said carriage, extending between an end located to control said magnetically actuable element, and an end proximate the rearward side of said carriage; means for moving said carriage in said translatory direction to and from a predetermined position; means, exterior to said carriage, operable to magnetize said pole piece in at least one sense; when said carriage is in said predetermined location.
 2. A device as claimed in claim 1 wherein said carriage mounts a plurality of said elements arranged in an array.
 3. A device as claimed in claim 1 wherein said means exterior to said carriage is a ''''write'''' head, designed and connected to provide magnetic flux in a selected sense to selected ones of said elements.
 4. A device as claimed in claim 1 wherein the rearward end of each said pole piece projects rearwardly from the carriage in a predetermined direction and wherein a ''''write'''' head is reciprocally movable between an extended and a retracted position relative to said carriage and carries an energizing coil corresponding to each said pole piece arranged, mounted and designed so that on location of said ''''write'''' head at its extended position and with said carriage in said predetermined position, an energizing coil surrounds at least some of said permanently magnetizable pole pieces and a location of said ''''write'''' head at its retracted position, clears the end of said permanently magnetizable pole piece to allow movement of said pole piece in a translation direction.
 5. A device as claimed in claim 1 wherein said write head is reciprocally mounted to move between an advanced and a retracted position relative to said carriage and mounts an energizing coil corresponding to each said pole piece, each said coil surrounding a core projecting therefrom in the direction of the advanced position, and wherein said write head and core are arranged so that on movement of the carriage to the forward position, flux generated in a ''''write'''' head core will magnetize the corresponding carriage pole piece.
 6. A device as claimed in claim 5 wherein said write head core is formed of reversibly permanently magnetizable material.
 7. A device as claimed in claim 5 wherein the ''''write'''' head mounts cores corresponding to each of said pole pieces and arranged so that in said predetermined position, each of said ''''write'''' head cores are located in close proximity to the respective pole pieces, whereby selective energization of certain of said coils in a predetermined sense creates flux in the corresponding cores and wherein the magnetic coupling between a corresponding core and pole piece is such that a substantial portion of flux from a ''''write'''' head core proceeds into the corresponding pole piece.
 8. A movable display including a carriage movable in a translation direction and mounting an array of elements each selectively movable by a corresponding magnetic field exterior to the element between two positions, said elements being adapted to provide, in a viewing direction transverse to said translation direction, contrasting visual effects, depending on which of the two positions is assumed by the element, a pole piece corresponding to each said element and exterior thereto, said pole piece being mounted on the carriage and having an end directed to provide a field at the corresponding element, and having a free end adjacent the side of said carriage opposite from that wherein viewing takes place; means for stopping said carriage at a predetermined position, and means, exterior to said carriage, when said carriage is stopped, operable at said predetermined position, for magnetizing selected ones of said pole pieces in a predetermined sense.
 9. Means as claimed in claim 8 wherein means are provided to cause said carriages to move in a closed path and wherein, at a location remote from the said write head, means are provided for ensuring that all pole pieces are magnetized in a predetermined sense.
 10. A movable display including: a carriage movable in the translation direction; and mounting an array of elements, each movable by a magnetic field exterior to the element between two positions; presentive in a forward direction, transverse to the viewing direction, said array, by selective positioning of the elements, being adapted to present said forward direction, contrasting visual effects, depending on which of the two positions is assumed by individual elements in the array; a pole piece mounted on said carriage corresponding to each said element, and having a free end adjacent the side of said carriage opposite to that where said elements are viewed; means for stopping said carriage in a predetermined position; And means, exterior to said carriage, operable at said predetermined position, for magnetizing selected ones of said pole pieces in a predetermined sense.
 11. Means as claimed in claim 4, wherein means are provided to cause said carriages to move in a closed path.
 12. Means as claimed in claim 4 wherein means are provided at a location spaced in the translation direction from said exterior magnetizing means, for ensuring that all said pole pieces are magnetized in a predetermined sense.
 13. Means as claimed in claim 5 wherein means are provided to cause said carriages to move in a closed path.
 14. Means as claimed in claim 8 wherein means are provided to cause said carriages to move in a closed path.
 15. Means as claimed in claim 5 wherein means are provided at a location spaced in the translation direction from said exterior magnetizing means, for ensuring that all said pole pieces are magnetized in a predetermined sense.
 16. Means as claimed in claim 8 wherein means are provided, at a location spaced in the translation direction from said exterior magnetizing means, for ensuring that all said pole pieces are magnetized in a predetermined sense.
 17. Means as claimed in claim 9 wherein means are provided, at a location spaced in the translation direction from said exterior magnetizing means, for ensuring that all said pole pieces are magnetized in a predetermined sense.
 18. Translating display apparatus using a carriage carrying a plurality of indication or display magnet-carrying elements, each movable with their magnets between two positions and designed to provide alternative contrasting visual indications corresponding to the two positions in a forward or viewing direction transverse to the translation direction, wherein the position from time to time of each element is determined by a corresponding pole piece mounted to translate with the element but exterior thereto, said pole pieces being permanently reversibly magnetizable and located to determine the position of the element in accord with the pole piece magnetization, and means for translating said element and pole piece in the translation direction, in combination with means, operable, at a predetermined location in the translation path, to magnetize, in a predetermined sense, the selected ones of said pole pieces. 